Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for attenuating coherent noise produced by man-made devices that are towed in water during a marine acquisition survey and/or interpolating the data based on multi-datasets.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.
During a seismic acquisition process, as shown in FIG. 1, a vessel 110 tows plural detectors 112, which are disposed along a cable 114. Cable 114 together with its corresponding detectors 112 are sometimes referred to, by those skilled in the art, as a streamer 116. Vessel 110 may tow plural streamers 116 at the same time. Streamers may be disposed horizontally, i.e., lie at a constant depth z1 relative to the ocean surface 118. Also, plural streamers 116 may form a constant angle (i.e., the streamers may be slanted) with respect to the ocean surface.
Still with reference to FIG. 1, vessel 110 may also tow a seismic source 120 configured to generate an acoustic wave 122a. Acoustic wave 122a propagates downward and penetrates the seafloor 124, eventually being reflected by a reflecting structure 126 (reflector R). Reflected acoustic wave 122b propagates upward and is detected by detector 112. For simplicity, FIG. 1 shows only two paths 122a corresponding to the acoustic wave. Parts of reflected acoustic wave 122b (primary) are recorded by various detectors 112 (recorded signals are called traces) while parts of reflected wave 122c pass detectors 112 and arrive at the water surface 118. Since the interface between the water and air is well approximated as a quasi-perfect reflector (i.e., the water surface acts as a mirror for acoustic waves), reflected wave 122c is reflected back toward detector 112 as shown by wave 122d in FIG. 1. Wave 122d is normally referred to as a ghost wave because it is due to a spurious reflection. Ghosts are also recorded by detector 112, but with a reverse polarity and a time lag relative to primary wave 122b if the detector is a hydrophone. The degenerative effect that ghost arrival has on seismic bandwidth and resolution is known. In essence, interference between primary and ghost arrivals causes notches, or gaps, in the frequency content recorded by detectors.
Recorded traces may be used to determine the subsurface (i.e., earth structure below water bottom 124) and to determine the position and presence of reflectors 126. However, ghosts disturb the accuracy of the final image of the subsurface and, for at least this reason, various methods exist for removing ghosts, i.e., deghosting, from the acquired seismic data.
In addition to ghosts, noise naturally occurring and noise produced by man-made devices associated with the seismic acquisition process also degrade the final image's accuracy. Some man-made devices are connected, for example, to towed streamers and they may be a constant, coherent source of noise. Examples of man-made devices generating unwanted but coherent noise include diverters, paravanes, tail buoys, birds and steering devices. These devices also cause drag and produce noise that travels down the streamer with a chevron shape, i.e., a symmetrical shape that may look like a wedge. Generally, noise from these man-made devices is linear in appearance. The noise's propagation along the cable may be dispersive. An example is illustrated in FIG. 2 for which the pressure data looks clean, but FIG. 3 shows the particle motion data 300 containing strong bird noise 302. Reference number 302 points to the bird noise, but it also points to the position of the bird along the streamer.
Noise 302 may be present at a range of frequencies, and traditional attempts to remove this noise often include frequency-wavenumber (FK) dip filtering or tau-p muting. Traditional approaches can be unsatisfactory because diffraction energy (which carries valuable seismic information) may be attenuated, the noise may be aliased, or an apex of the noise (which should be removed) may be left or generate artefacts.
Thus, there is a need for a new process to attenuate such noise where the position of the apex is known, and this process should not be limited to bird noise, but be applicable to any coherent noise with chevron appearance.